The present invention relates to a pneumatic tire and more particularly, to an improved bead construction for a pneumatic tubeless tire, particularly employing a bias-ply carcass of steel cord.
Commonly, there are two types of pneumatic tires, i.e., those using inner tubes and the tubeless type which does not use inner tubes. In the latter tubeless type which is widely applied to recent motor vehicles, the tire is mounted on a wheel rim in such a manner that air is retained between the rim and tire casing and the tire bead is so constructed that it seals tightly against the rim flange to retain air pressure when the tire is inflated.
In FIG. 1 showing a construction of a conventional pneumatic tubeless tire as described above, the tire or tire cover 1 made of rubber or similar material generally includes a tread portion 1t, a pair of opposed side wall portions 1d contiguous to both sides of the tread portion 1t, and pair of bead portions 2 in which bead cores 3A are respectively embedded.
The tire cover 1 further includes a bias-ply carcass portion 4 facing a pneumatic chamber 1p defined in the tire cover 1 and composed, for example, of at least two layers of steel cord adjoining each other, and an inner layer 8 of rubber material further deposited on the bias-ply carcass portion 4 in a known manner. The carcass portion 4 extends between the bead portions 2 with opposite ends thereof inwardly turned up or folded back around the respective bead cores 3A and secured in positions within the bead portions 2. Thus a substantially intermediate portion of the carcass portion 4 is provided which extends immediately below the tread portion 1t.
More specifically in FIG. 2, each of the bead portions 2 includes a base portion 2a which is inclined at an angle .theta. of approximately 15.degree. with respect to a line c parallel to an axial line of a wheel (not shown), and the bead core 3A formed by a bundle of bead steel wires and a suitable filling member to have an approximately hexagonal cross section and embedded in the bead portion 2 in such a manner that one lower side 3Aa of the hexagonal cross section is approximately parallel to the base portion 2a, while an end ply edge 4a of the carcass portion 4 is turned up around the periphery of the bead core 3A, with a filler 7, for example, of steel cord being further applied onto such a turned up portion of the carcass 4 as shown. Upon fitting of the base portion 2a of the bead portion 2 onto a tire seat portion 5 of a wheel rim 6 having the same inclination of the angle .theta. as that of the base portion 2a, the driving force of the wheel (not shown) is transmitted to the tire 1 through frictional force between the base portion 2a of the bead portion 2 and the tire seat portion 5 of the wheel rim 6.
In the known arrangement as described above, during the use of the tire 1, the carcass portion 4 and filler 7 are pulled or subjected to tension in the directions shown by the arrows due to internal air pressure P applied onto the inner surface of the tire 1 upon inflation of said tire 1, and the steel cord portions of the carcass 4 and filler 7 tend to be separated from the rubber material of the tire 1 particularly in the vicinity of the end ply edge 4a of the carcass 4 and an edge 7a of the filler 7, with the base portion 2a of the bead portion 2 being consequently deformed as shown by a chain line in FIG. 2. The tire 1 whose bead base portions 2a are deformed as described above is extremely difficult to seal against the internal air pressure P, during re-mounting thereof onto the wheel rim 6 after once detached from said rim 6, and if the deformation is excessive, there are cases where slipping takes place between the wheel rim 6 and the tire 1. Accordingly, in order to reduce the influence by the above described pulling due to the internal air pressure P over the end edge 4a of the carcass portion 4 and the edge 7a of the filler 7 as far as possible, and also to minimize the deformation of the bead base portion 2 a, it is advantageous to decrease the internal diameter relating to the of the bead core 3A i.e., the internal diameter of the bead cord ring and simultaneously to arrange the particular lower side 3Aa of the hexagonal cross section of the bead core 3A as wide as practicable.
On the other hand, during the vulcanization and molding in the manufacture of the tire 1 in a known manner, the external diameter of a raw tire (not shown) before such vulcanization and molding is made smaller than the inner diameter of the tire mold (not shown) by 2 to 5% so as to be expanded to a corresponding extent by the internal air pressure P before the vulcanization to achieve a close contact between the tire mold and the raw tire for proper molding, and in the above case, the bias-ply carcass portion 4 is also pulled by approximately 5 mm in the direction of the arrow in FIG. 2, with consequent lowering in the position of the end edge 4a of the carcass portion 4 by a corresponding extent. If such movement of the carcass portion 4 is not smoothly effected during the manufacture of the tire 1, not only the position of the end ply edge 4a is not maintained uniform throughout the tire on the whole, but the inner layer 8 to be formed on the bias-ply carcass portion 4 tends to pass through the carcass portion 4 into the rubber material of the tire cover 1 toward the other side of such carcass portion 4, with a possibility that the carcass portion 4 is exposed to the inner side of the tire 1 in extreme cases. Therefore, for causing the carcass portion 4 to be slipped or displaced smoothly around the bead core 3A during the manufacture of the tire 1, it is preferable that the bead core 3A has a cross section close to a circular shape.
As is seen from the foregoing description, conditions contrary to each other are required during the use and in the manufacture of the tire 1.
For satisfying such requirements as described above, there may be employed, during the molding of the raw tire, a composite bead core 11 preliminarily prepared by applying a filling rubber member 9 of arcuate cross section onto the upper portion of the bead core 3A and lapping a rubber coated fabric 10 and the like around the core 3A after application of zinc stearate, etc. having mold releasing effect onto said core 3A, so that the carcass portion 4 is caused to slip in close contact around the outer periphery of the rubber coated fabric 10 for smooth displacement of said carcass portion 4 in the direction of the arrow in FIG. 2. The known arrangement as described above, however, has such a disadvantage that a considerable time or man-hour is required for the application of the filling rubber member 9, thus resulting in high cost in the manufacture of the tire. Alternatively, as shown in FIG. 3, there may be employed a bead core 3B having a rubber layer 12 preliminarily molded therearound, for example, by press work and the like, with the one lower edge 3Ba of the hexagonal cross section being arranged approximately in parallel with the base portion 2a for use during the molding of the raw tire. However, such an arrangement also requires extra man-hours for the formation of the rubber layer 12 around the bead core 3B by pressing, with a consequent increase in cost.